The present invention relates generally to digital systems, and particularly to obtaining the identification or address of a board which may be plugged into a backplane.
The present invention is an improved method and apparatus for obtaining board address information for boards which are plugged into slots in a backplane. Typical backplanes may have twenty or more slots, requiring five or more bits to identify the binary address for each board. The board interface to the backplane will include one reserved pin for each address bit. The pins typically have pull-up resistors, and the backplane connectors have various bits grounded on each slot corresponding to the slot address. As an example, slot 0 would have all five pins grounded, representing binary xe2x80x9c0xe2x80x9d, while slot 1 would have the four most significant bits grounded and the least significant bit floating, representing binary xe2x80x9c1.xe2x80x9d Each slot in the backplane thus has its own unique binary address.
A problem arises when there are not enough spare pins on the backplane to implement this addressing scheme. Alternatively, the device that reads the address may not have sufficient spare pins. Thus, it is desired to provide a method and apparatus for providing backplane slot address information using a single pin.
In summary, the present invention is an apparatus and method for providing board address information for a board in a backplane using a single pin in the backplane slot. A unique address for a board in a backplane slot is identified by an analog address signal, which can be communicated on a single line and therefore requires only one pin in the backplane slot or in the device which will read the address information. The analog address signal is a function of a component, such as a resistor or a capacitor, coupled to the address pin in the backplane slot. A predefined characteristic of the component, such as the resistance or the capacitance, is unique for each slot. The component characteristic is converted into a corresponding characteristic of the analog address signal, such as the voltage, current, frequency or the slope of a voltage or current ramp, so that the analog address signal is measurably different for every slot.